1. Field of the Invention
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
2. Related Art Statement
In recent years, most vehicles are equipped with a so-called power steering gear, and various type power steering unit, such as a hydraulically or electrically operated type power steering unit has been designed to assist the steering power of the vehicle. In these electric power steering units, as one for applicable to rack and pinion type steering systems, a unit which the steering assisting power is obtained by an electric motor arranged coaxially with a rack-shaft is known. FIG. 5 is a cross-sectional view showing the structure of a conventional electric power steering unit, and FIG. 6 is an enlarged view showing the main part of the unit shown in FIG. 5.
An electric power steering unit shown in FIG. 5, comprises an electric motor 52 arranged coaxially with a rack-shaft 51 so that the steering assisting power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53. Then, the guiding wheels of the vehicle can be turned by utilizing both the manual steering power of the driver and the steering assisting power.
The rack-shaft 51 is linked to the guiding wheels typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel (which hereinafter may be referred to as "the handle") by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 5 as the steering operation by the driver. The electric motor 52 has a cylindrical yoke 55 containing coaxially therein a cylindrical armature shaft 56 and a field device 57 and is fed with power from a power supply section 58. The field device 57 comprises magnets 59 arranged on the inner peripheral portion of the yoke 55 and an armature core 60 arranged on the outer peripheral portion of the armature shaft 56. The rotary power generated by the electric motor 52 is transmitted to the rack-shaft 51 and the power is converted into the reciprocatively movement by way of a ball screw mechanism 53 arranged at the left end of the armature shaft 56 in FIG. 5, so that steering power is assisted.
Meanwhile, in the power steering device shown in FIG. 5, the ball screw mechanism 53 comprises a nut section 62 and a screw section 63, and the nut 62 and the screw 63 are coupled without backlash by balls 64. The nut 62 of the ball screw mechanism 53 is supported to be rotatable in the housing 61 by an angular bearing 65.
In this case, a spacer 71 for arranging the angular bearing 65 and the nut 62 to be coaxial with each other is provided therebetween as shown in FIG. 6. The inner ring side of the angular bearing 65 is fixed by an inner ring fixing ring 74 screwed on an outer circumferential portion of the spacer 71 and by the flange 75 of the spacer 71, and the outer ring side of the angular bearing 65 is fixed by an outer ring fixing ring 72 screwed on an inner circumferential portion of the housing 61 and by a stepped section 73 of the housing 61. Meanwhile, the nut 62 is contained inside the spacer 71, and the nut 62 is fixed by a nut fixing ring 76 screwed on an inner circumferential portion of the spacer 71. Further, in this manner, the nut 62 is rotatably supported and fixed in the housing 61 through the spacer 71 by the angular bearing 65.
However, since the angular bearing 65 and the nut 62 are separated from each other in a conventional power steering unit, the spacer 71 as described above is required in order to arrange the bearing 65 and the nut 62 to be coaxial with each other. Therefore, there is a problem that the number of components is increased to raise the costs of products. Hence, there recently was a proposal for an unit in which the nut 62 and the inner ring of the angular bearing 65 are integrated with each other, like the electric power steering unit according to Japanese Patent Application Laid Open No. 9-104351. In this application, the ball screw nut is supported in the housing without using the spacer 71 to reduce the number of components and the size of the product.
Meanwhile, as shown in FIG. 7, there is another conventional power steering unit in which one end side of a ball screw nut 81 is received by a thrust bearing 82 and the other end side thereof is supported by an angular bearing 83. In this case, a pre-load pressure is applied in the axial direction to a pressure member 89 and a fixing screw ring 90, and the nut 81 is contained and maintained in a lower housing 88 under the pre-load pressure. In addition, a circulation passage 84 is formed inside the nut 81, and balls 85 move in the passage 84 thereby smoothly converting rotation of the nut 81 into movement in the lateral direction of a rack shaft 87.
However, since this power steering unit is arranged such that a pre-load pressure is applied to the angular bearing 82 when the nut 81 is fixed into the housing, there is a problem that this pressure is transmitted to balls 85 through the nut 81 and a stress is caused at the balls 85 against the pressure thereby deforming the balls 85. There also is a problem that a force from the angular bearing 82 is applied to the passage 84 for circulating the balls 85, so that the passage 84 is deformed and the rolling of the balls 85 is disturbed.